The provisional systems for digital terrestrial television broadcasts and digital terrestrial sound broadcasts announced by the Japanese Telecommunications Technology Council are systems having compatibility in broadcasts. They construct OFDM (orthogonal frequency division multiplexing) modulated basic transmission units referred to as “segments” from the about 429 kHz obtained by dividing the frequency band of 6 MHz allocated to television channels in Japan into 14 and use the segments for transmission. Three modes of the OFDM modulated segment are defined, that is, ones containing 108, 216, and 432 OFDM carriers.
It is proposed to construct a transmission signal by using 13 segments for digital terrestrial television broadcasts and construct a transmission signal by using one or three segments for digital terrestrial sound broadcasts.
The Digital Terrestrial Broadcast Roundtable Conference, an advisory body of the Ministry of Posts and Telecommunications, has proposed for future digital terrestrial broadcasts in Japan to broadcast digital terrestrial television broadcasts by using the UHF (ultra high frequency) band which is being used for television broadcasts at present and to broadcast digital terrestrial sound broadcasts (radio broadcasts) by using the VHF (very high frequency) band which is being used for television broadcasts at present.
The band being used as an intermediate frequency in a television receiver of terrestrial analog television broadcasts at present is from 54 MHz to 60 MHz. It has been considered that use of the intermediate frequency of current terrestrial analog television broadcasts in digital terrestrial television broadcasts and digital terrestrial sound broadcasts as well would be advantageous from the viewpoint of avoiding image interference. Namely, it has been proposed to give a bandwidth necessary for having 57 MHz as the center frequency and intermediate frequency characteristics having selectivity of the attenuation characteristics etc. and demodulate this.
As explained above, it has been proposed to conduct digital terrestrial sound broadcasts in the VHF band. Further, it has been demanded to remove interference with existing analog television broadcasts. Also, since 13 segments can be transmitted in one channel's bandwidth which has been used for analog television broadcasts, when a digital terrestrial sound broadcast program comprises a one-segment transmission signal, as much as 13 channels' worth of programs can be included. Accordingly, at the receiving apparatus side, in addition to setting the channel number which the user wants to receive, it is necessary to set which segment in the channel to receive.
For this reason, the relative position of the frequency with the signal of the adjacent analog television broadcast differs depending upon which segment in the channel the receiving apparatus receives. In general, the closer the received signal to the adjacent signal, the more susceptible it probably is to interference, so when segments located at the ends of a channel are received, correct demodulation is the most difficult, in other words, the influence from the adjacent signal is the largest.
As explained above, however, as a demodulation scheme using an intermediate frequency having 57 MHz as a center frequency taking into consideration the band of the center frequency of current television receivers, a first technique of directly demodulating the intermediate frequency to a baseband and a second technique of converting the intermediate frequency to a further other frequency, then demodulating it can be considered.
When using the first technique and performing the processing in an analog manner, there are the disadvantages that, for example, it is hard to obtain stability in orthogonality for generation of orthogonal carriers, a direct current offset occurs due to the incompleteness of an analog demodulation use multiplier circuit, and two A/D (analog/digital) conversion circuits become necessary for digitalization of the baseband signal. Also, when using the first technique and performing the processing in a digital manner, the orthogonal carriers and clock frequencies are not also compatible, so there was the problem of the size of the circuit becoming larger.
When using the second technique, the intermediate frequency of the 57 MHz band is further converted to a frequency which is easily digitally demodulated, so it is possible to stably operate the digital demodulation circuit and only one A/D conversion circuit need be prepared. However, since the intermediate frequency of the 57 MHz band is further converted to another frequency, there arises a new problem relating to image suppression due to this second frequency conversion. Particularly, an adjacent picture carrier or sound carrier may become an image and leak into the desired signal. It is necessary to prevent this.